1. Field of the Invention
This invention relates to apparatus for detecting arcing faults in ac electrical systems, and, in particular, to such apparatus which tracks and is sensitive to the amplitude of the step changes in current produced by arcing faults.
2. Background Information
It has become recognized that conventional overcurrent protection devices for ac electrical systems such as circuit breakers and overload relays typically do not respond to arcing faults. Such faults often have a high resistance so that the fault current is below the instantaneous trip current of such protection devices. Arcing faults in ac systems also tend to be intermittent so that they do not generate the time integrated values of current needed to activate the delayed trip feature in the typical circuit protection.
A common approach to arcing fault detection recognizes that the arc current is considerably distorted from a pure sine wave. For instance, it contains considerable high frequency noise. In addition, there can be intervals of no current and/or periods of short circuit current. Devices which combine various of these characteristics of arc current have been proposed for arc detectors. Some of them require microcomputers to apply the various criteria to the current wave form. Generally, this makes such detectors too expensive for widespread use. In addition, many common loads have capacitors or transformers on their inputs which filter the high frequency noise.
Another approach to arc fault detection relies on the fact that an arc between spaced conductors or a gap in a conductor can only be struck when the voltage rises to the break down voltage across the space or gap. Thus, these detectors respond to the step increase in current produced by arc initiation. Such a detector is described in U.S. Pat. No. 5,224,006. In order to avoid false trips produced by the inrush currents at turn on of some typical loads, another detector of this type described in U.S. Pat. No. 5,940,256 looks for a plurality of step increases in current within a certain time interval characteristic of an arcing fault which repetitively strikes and is interrupted. A further improvement on this type of detector, which generates a time attenuated integration of pulses representative of step increases in current such as those generated by arcing faults, is described in U.S. Pat. No. 5,691,869.
Any arcing fault detector must be able to discriminate between a true arcing fault and wave form distortion produced by normal loads. One such load is a dimmer. A dimmer phases back the half cycles of the ac voltage creating repetitive step increases in current. The above-described arcing fault detectors which respond to a plurality of step increases in current must have their sensitivities reduced to avoid a false response to dimmners. Adding to the problem, a dimmer energizing a tungsten lamp can generate inrush pulses with an amplitude ten times steady state current when the filament is cold.
U.S. Pat. No. 5,818,237 discloses an arcing fault detector which responds to low amplitude arcing faults without generating false indications in response to a dimmer or the spikes generated by turn-on of a cold tungsten bulb. This detector looks for randomness in the envelope of a sensed current signal containing pulses for the step increases in current caused by the striking of an arc. In one such detector, the envelope of the pulse signal is tracked by a first tracking circuit with a first time constant to generate a first tracking signal, and a second tracking circuit tracking the sensed current signal with a second time constant which is shorter than the first time constant to generate a second tracking signal. An output signal is generated when the second tracking signal decays to a predetermined fraction of the first tracking signal. As a dimmer generates a step increase in current on each half cycle, the time constants and the fraction can be selected such that the second tracking signal never falls to the predetermined fraction of the first tracking signal in response to a dimmer. On the other hand, the random repetition of step increases in current produced by an arcing fault results in intervals where the second tracking signal will fall to the predetermined fraction of the first tracking signal and produce an output. The second tracking signal must fall to the predetermined fraction of the first tracking signal a selected number of times within a prescribed time interval before an indication of an arcing fault is generated in order to distinguish over singular events. In another embodiment of this detector, envelope detection is combined with generation of a time attenuated accumulation of pulses generated by the step increases in current caused by striking of an arc as in the detector of U.S. Pat. No. 5,691,869. The sensitivity to arcing faults is increased by additionally incrementing the time attenuated accumulation by adding a pulse to the accumulation each time the fast envelope tracking signal falls to the predetermined fraction of the slow envelope tracking signal.
The detector of U.S. Pat. No. 5,818,237 generates fixed pulses when the second tracking signal falls to the selected fraction of the first tracking signal. While this approach works reasonably well, there is room for improvement. In particular, there is a desire to increase the sensitivity of envelope detection tpe arc detectors to low current arcs without sacrificing reaction time for the higher current, more dangerous arcing faults.